COMMUTATIVE POST-LIE ALGEBRA STRUCTURES ON LIE ALGEBRAS DIETRICH BURDE AND WOLFGANG ALEXANDER MOENS Abstract. We show that any CPA-structure (commutative post-Lie algebra structure) on a perfect Lie algebra is trivial. Furthermore we give a general decomposition of inner CPA- structures, and classify all CPA-structures on complete Lie algebras. As a special case we obtain the CPA-structures of parabolic subalgebras of semisimple Lie algebras. 1. Introduction Post-Lie algebras have been introduced by Vallette [22] in connection with the homology of partition posets and the study of Koszul operads. Loday [17] studied pre-Lie algebras and post- Lie algebras within the context of algebraic operad triples. We rediscovered post-Lie algebras as a natural common generalization of pre-Lie algebras [13, 15, 20, 4, 5, 6] and LR-algebras [8, 9] in the geometric context of nil-affine actions of Lie groups. We then studied post-Lie alge- bra structures in general, motivated by the importance of pre-Lie algebras in geometry, and in connection with generalized Lie algebra derivations [7, 10, 11, 12]. In particular, the existence question of post-Lie algebra structures on a given pair of Lie algebras turned out to be very interesting and quite challenging. But even if existence is clear the question remains how many structures are possible. In [12] we introduced a special class of post-Lie algebra structures, namely commutative ones. We conjectured that any commutative post-Lie algebra structure, in short CPA-structure, on a complex, perfect Lie algebra is trivial. For several special cases we already proved the conjecture in [12], but the general case remained open. One main result of this article here is a full proof of this conjecture, see Theorem 3:3. Furthermore we also study inner CPA-structures and give a classification of CPA-structures on parabolic subalgebras of semisimple Lie algebras. In section 2 we study ideals of CPA-structures, non-degenerate and inner CPA-structures. In particular we show that any CPA-structure on a complete Lie algebra is inner. We give a general decomposition of inner CPA-structures, see Theorem 2:14. This implies, among other things, that any Lie algebra g admitting a non-degenerate inner CPA-structure is metabelian, i.e., satisfies [[g; g]; [g; g]] = 0. In section 3 we prove the above conjecture and generalize the result to perfect subalgebras of arbitrary Lie algebras in Theorem 3:4. This also implies that any Lie algebra admitting a non-degenerate CPA-structure is solvable. Conversely we show that any non-trivial solvable Lie algebra admits a non-trivial CPA-structure. In section 4 we classify CPA-structures on complete Lie algebras satisfying a certain techni- cal condition. As an application we obtain all CPA-structures on parabolic subalgebras of semisimple Lie algebras. Date: August 19, 2016. 2000 Mathematics Subject Classification. Primary 17B30, 17D25. Key words and phrases. Post-Lie algebra, Pre-Lie algebra. The authors acknowledge support by the Austrian Science Foundation FWF, grant P28079 and grant J3371. 1 2 D. BURDE AND W. MOENS 2. Preliminaries Let K always denote a field of characteristic zero. Post-Lie algebra structures on pairs of Lie algebras (g; n) over K are defined as follows [10]: Definition 2.1. Let g = (V; [ ; ]) and n = (V; f ; g) be two Lie brackets on a vector space V over K.A post-Lie algebra structure on the pair (g; n) is a K-bilinear product x · y satisfying the identities: (1) x · y − y · x = [x; y] − fx; yg [x; y] · z = x · (y · z) − y · (x · z)(2) (3) x · fy; zg = fx · y; zg + fy; x · zg for all x; y; z 2 V . Define by L(x)(y) = x · y and R(x)(y) = y · x the left respectively right multiplication operators of the algebra A = (V; ·). By (3), all L(x) are derivations of the Lie algebra (V; f; g). Moreover, by (2), the left multiplication L: g ! Der(n) ⊆ End(V ); x 7! L(x) is a linear representation of g. A particular case of a post-Lie algebra structure arises if the algebra A = (V; ·) is commutative, i.e., if x · y = y · x is satisfied for all x; y 2 V . Then the two Lie brackets [x; y] = fx; yg coincide, and we obtain a commutative algebra structure on V associated with only one Lie algebra [12]. Definition 2.2. A commutative post-Lie algebra structure, or CPA-structure on a Lie algebra g is a K-bilinear product x · y satisfying the identities: (4) x · y = y · x [x; y] · z = x · (y · z) − y · (x · z)(5) x · [y; z] = [x · y; z] + [y; x · z](6) for all x; y; z 2 V . The associated algebra A = (V; ·) is called a CPA. There is always the trivial CPA-structure on g, given by x·y = 0 for all x; y 2 g. However, in general it is not obvious whether or not a given Lie algebra admits a non-trivial CPA-structure. For abelian Lie algebras, CPA-structures correspond to commutative associative algebras: Example 2.3. Suppose that (A; ·) is a CPA-structure on an abelian Lie algebra g. Then A is commutative and associative. Indeed, using (4), (5) and [x; y] = 0 we have x · (z · y) = x · (y · z) = y · (x · z) = (x · z) · y for all x; y; z 2 g. It is easy to see that there are examples only admitting trivial CPA-structures: Example 2.4. Every CPA-structure on sl2(K) is trivial. This follows from a direct computation, but also holds true more generally for every semisim- ple Lie algebra, see Proposition 3:1. One main aim of this paper is to show that this is even true for all perfect Lie algebras, see Theorem 3:3. POST-LIE ALGEBRA STRUCTURES 3 Definition 2.5. A CPA-structure (A; ·) on g is called nondegenerate if the annihilator AnnA = ker(L) = fx 2 g j L(x) = 0g is trivial. Note that AnnA is an ideal of the CPA as well as an ideal of the Lie algebra. Here a subspace I of V is an algebra ideal if A · I ⊆ I, and a Lie algebra ideal if [g;I] ⊆ I. An ideal is defined to be an ideal for both A and g. Let [x1; : : : ; xn] := [x1; [x2; [x3; : : : ; xn]]] ··· ] and I[n] := [I; [I; [I; ··· ]]] ··· ] for an ideal I. Proposition 2.6. Suppose that (A; ·) is a CPA-structure on g. Then there exists an ideal I1 such that [k] (1) I1 ⊆ AnnA ⊆ I1 for all k large enough. (2) The CPA-structure on g=I1 is nondegenerate. Proof. Define an ascending chain of ideals In by I0 = 0 and In = fx 2 A j x · A ⊆ In−1g for n ≥ 1. We have I1 = AnnL and each In is indeed an ideal because of In · A ⊆ In−1 ⊆ In, and [In; g] · A ⊆ In · (g · A) + g · (In · A) ⊆ In · A + g · In−1 ⊆ In−1: So for x 2 [In; g] and a 2 A we have x · a 2 In−1, hence x 2 In. Since g is finite-dimensional, this chain stabilizes, i.e., there exists a minimal k such that Ik = I` for all ` ≥ k. Then define I1 := Ik. By construction we have A · I1 = 0, A · (A · I2) ⊆ A · I1 = 0, etc., so that right-associative products in I1 of length at least k + 1 vanish. Using (5) we have [x1; : : : ; xn−1; xn] · z = [x1; : : : ; xn−1] · (xn · z) − xn · ([x1; : : : ; xn−1] · z) for all x1; : : : ; xn; z 2 V . By induction we see that the elements [x1; : : : ; xn] · z are spanned by the right-associative elements xπ(1) · xπ(2) ··· xπ(n) · z, where π runs over all permutations in Sn. [k+1] [k+1] This yields I1 ·g = 0, and hence I1 ⊆ AnnA. We also have AnnA = I1 ⊆ I1. Furthermore x · g ⊆ I1 implies x 2 I1, so that the induced CPA-structure on g=I1 is nondegenerate. Note that I1 is in fact the minimal ideal with this property. Definition 2.7. A CPA-structure on g is called weakly inner, if there is a ' 2 End(V ) such that the algebra product is given by x · y = ['(x); y]: It is called inner, if in addition ' is a Lie algebra homomorphism, i.e., ' 2 End(g). In terms of operators this means that we have L(x) = ad('(x)) for all x 2 V . We have ker(') ⊆ ker(L) with equality for Z(g) = 0. Lemma 2.8. Let g be a Lie algebra with trivial center. Then any weakly inner CPA-structure on g is inner. Proof. A product x · y = ['(x); y] with some ' 2 End(V ) defines a CPA-structure on g, if and only if ['(x); y] = ['(y); x] [['(x);'(y)]; z] = ['([x; y]); z] 4 D. BURDE AND W. MOENS for all x; y; z 2 g. In case that Z(g) = 0 the last condition says that ' is a Lie algebra homomorphism. Corollary 2.9. Let g be a complete Lie algebra. Then any CPA-structure on g is inner. Proof. By definition we have Der(g) = ad(g) and Z(g) = 0. Hence L(x) 2 Der(g) implies that L(x) = ad('(x)) for some ' 2 End(g). In general not all CPA-structures on a Lie algebra are inner or weakly inner. This is trivially the case for abelian Lie algebras, which do admit nonzero CPA-structures, which cannot be weakly inner.